824 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ß :.:'. ..• '.• ,---? ........ .:,,: . • 'I• ß :.• .{ :. • ":!!"'"::: *•" "' - '"'¾'! "•"• ' ¾% ' ,•...•, ..... ?..• •.... .. Figure 5. Tooth mount The cavity area of the dental stone is coated with petrolatum and a 5-ram length of 10-ram i.d. polyethylene tubing is placed in the center of the cavity. Acrylic monomer and polymer are added around the tubing to a depth of about 3-4 mm. When the acrylic hardens to the dough stage, 6 carious-free crowns obtained by cutting mechanically extracted teeth at the cemento- enamel junction are spaced equally around the polyethylene tubing. The crowns should be immersed to a depth of 1-2 mm to achieve adequate adhe- sion and overall uniformity of crown height. After the acrylic has hardened (20 rain), the "acrylic donut" with the embedded crowns is removed and the acrylic is coated with a thin layer of hard inlay wax. The excess wax is carved from around the necks of the crowns. A suitable instrument such as a dental excavation spoon is heated with a small flame and wax is flowed into the oc- clusal pits and fissures of the mounted crowns. Throughout the procedure, care should be taken to minimize the drying of the crowns. Dissolution Apparatus (Fig. 6) A 180-ml tall form beaker (Coming #1140) is prepared to support the wax-covered donut-like mounts by making 4 equidistant 5-mm indentations about 30 mm from the bottom. This can be done by heating the area with an oxygen-air torch and pushing a metal probe such as the tip of a rat tail file into the beaker wall to the desired depth. This beaker is fitted with a glass stirrer (Sargent #5-76667-A) cut to a shaft length of about 130 mm and a

THERAPEUTIC DENTIFRICES 825 ........ •o .•...- Figure 6. Assembled dissolution unit constant-speed 1800-rpm motor.* The glass stirring rod is attached to the mo- tor with about 45 rain lcngth of 6-ram i.d., 16-ram o.d. pressure rubber tubing. It is convenient to make up dissolution units in sets of four which are mounted in a 37øC constant temperature bath (Fig. 7). Lactate Buffer To a 2-1. volumetric flask is transferred 211 g (2 moles) of 85.2% lactic acid, analytical reagent grade, and 400 ml of water is added. Next, 84 g (2.07 moles) of sodium hydroxide, analytical reagent grade, is dissolved in 600 ml of water and transferred completely to the lactic acid-containing volumetric flask. If the pH of the solution is not above 10.5, i g of sodium hydroxide is added and the pH is measured after standing 30 min. The solution is then diluted to 1 1. with water. Next, 211 g of 85.2% lactic acid reagent is transferred to a 2-1. volumetric flask and diluted to the mark with water to give 1M lactic acid. To a 3-1. beaker is transferred the 2 1. of 1M sodium lactate solution and about 425 ml of the 1M lactic acid solution is added to bring the pH of the mixture to 4.50. It is preferable to age this solution about a month or more to reduce the lactic acid polymer coutent. The 1M lactate buffer should be diluted *Dayton •oo H.P. shaded pole #4K903, Dayton Motor Co., Dayton, Ohio.